Until recently, it has been generally accepted that growth hormone (GH) and prolactin are invariably secreted by separate and distinct cell types within the pituitary gland. However, experiments conducted over the past two years in my laboratory have necessitated a reevaluation of this view. Utilizing reverse hemolytic plaque assays, that enable the detection of hormone release at the single cell level, we have identified individual pituitary cells that simultaneously release both GH and prolactin. These dual cells, designated mammosomatotropes, account for approximately one-third of all GH and/or prolactin secretors in pituitary cultures derived from mature and neonatal rats of both sexes. The long-term objective of the proposed study is to delineate the secretory characteristics of mammosomatotropes and then to clarify the functional relationship between these dual hormone secretors and those cells that release only one hormone or the other. In pursuit of this objective, we plan to address two specific questions: 1) Does the control of hormone release from mammosomatotropes differ from that of classical mammotropes and somatotropes? and 2) Are GH and prolactin cells functionally interconvertible? This will be accomplished, in large part, by first exposing cultures of pituitary cells to various experimental treatments and then by using plaque assays to monitor the effects on hormone release by cells that secrete GH, prolactin, or both hormones. It is anticipated that the proposed studies will provide valuable information toward our basic understanding of GH and prolactin regulatory processes. Such knowledge is essential for identifying the causes of, and developing treatments for, GH and/or prolactin secreting adenomas of the pituitary.